Water-soluble polyamides have long been known, and water-soluble polyamides having various structures have been synthesized. For example, polyamides containing a sulfonic acid group or a salt thereof, polyamides containing an ammonium salt type of nitrogen atom, and polyamides containing an ether bond have been synthesized and used as water-soluble polyamides.
These water-soluble polyamides have excellent abrasion resistance, impact resistance, transparency and the like which are characteristic of polyamides. Furthermore, for the application of the water-soluble polyamides, an inexpensive and nontoxic solvent such as water can be used, and it is not necessary to use organic solvents which may cause environmental or pollution problems. Thus, such polyamides have been used in various applications.
Compositions comprising such water-soluble polyamides, polymerizable vinyl monomers, and sensitizers form light-sensitive resin compositions, and are used as printing materials as described, for example, in Japanese patent publication Nos. 43565/1974, 39846/1976 and 2082/1978. A light-sensitive resin plate is produced by bonding a sheet produced from such a light-sensitive resin composition onto a metal plate or film. When the light-sensitive resin plate is brought into close contact with a developed negative film containing an image pattern and is exposed to light, such as ultraviolet rays, through the negative film, polymerizable vinyl monomers are polymerized and insolublized (also referred to as "cross-linked" or "cured") at exposed areas, whereas at unexposed areas, they remain unpolymerized. Therefore, on washing the light-sensitive resin plate with water, the unexposed areas are dissolved and a resin relief pattern is obtained. In general, resin reliefs produced using such compositions composed mainly of polyamides have excellent abrasion resistance and affinity to ink which are characteristic of polyamides, and thus they have been widely used as printing materials having excellent properties in the filed of printing.
Light-sensitive resin plates produced using the foregoing water-soluble polyamides, however, suffer from several disadvantages. For example, when the light-sensitive resin plate is exposed to light and the unpolymerized areas are removed by washing with water to provide the resin relief pattern, the water penetrates into the polymerized areas, resulting in swelling of the cured areas. Particularly, for lines having a width of about 50.mu. or less, therefore, deformation of the line, and, in some cases, disappearance of the line, occur. Furthermore, when the resin relief thus obtained is immediately used in printing, printed letters and lines are broader than the originals, that is, extended, because the cured areas are also swollen with water. Additionally, the ability to transfer ink is deteriorated. Thus, in some cases, satisfactory printed letters or lines cannot be obtained. In order to overcome this problem, it is necessary to dry the resin relief at a high temperature for a long period of time. This is not desirable from the viewpoint of efficiency or productivity.
Compositions comprising water-soluble polyamides and reactive compounds, such as melamine resins, urea resins, epoxy compounds, phenol compounds, and methylol compounds, can be used as water-based paints or adhesives. These compositions are cured, e.g., by application of heat. Such cured products, however, are swelled with water when they come into contact with water, even after such curing, as in the case of the light-sensitive resin plate as described above. This leads to a reduction in the performance as a paint or adhesive, and imposes limitations on the usefulness thereof.
Vinyl alcohol-based polymers, such as polyvinyl alcohol and its derivatives which are obtained by partial or full saponification of polyvinyl acetate, have been used as fibers, pastes, adhesives, and so forth.
Light-sensitive resin compositions prepared by mixing such vinyl alcohol-based polymers with polymerizable vinyl monomers and sensitizers have been widely used in the field of printing as in the case of the foregoing light-sensitive polyamide resin compositions. Particularly, in view of the fact that they provide a layer having a high surface hardness, large amounts of light-sensitive resin compositions have been used as materials for the production of pattern plates. Resin plates produced from such resin compositions, however, have a severe disadvantage in that the resin plates exhibit breaking or peeling, particularly during winter months when the temperature and humidity are low, because of poor flexibility and impact resistance.
As is well known in the art, vinyl alcohol-based polymers have also been used heretofore as adhesives. Since the vinyl alcohol-based polymer is not generally easily melted by heat owing to the strong hydrogen bond formed between hydroxyl groups which are contained in the vinyl alcohol-based polymer, it is usually dissolved in water as a solvent to provide an adhesive. The thus-obtained adhesive, however, has the disadvantage that a drying step is required after the coating of the adhesive. This produces problems in operation and imposes limitations on the usefulness.
Recently, in place of organic solvent type of adhesives which have been less employed in view of their toxicity, so-called hot melt type of adhesives having good heat-sealability, such as an ethylene-vinyl acetate copolymer and a polyamide, have been mainly used. These hot melt type of adhesive, however, suffer from several disadvantages. For example, difficulties are encountered in recovering pulp from waste paper which is bonded with these hot melt types of adhesives, because the adhesive such as ethylene-vinyl acetate copolymer and a general polyamide is not soluble in water. Thus, conventional hot melt types of adhesives are limited in their usefulness.
Under such circumstances, adhesives having both water-solubility and hot meltability have been developed. For example, Japanese Patent Publication No. 9611/1974 discloses a water-soluble hot melt type of adhesive wherein a vinyl alcohol-based polymer which is available at low cost is used as a major component and pentaerythritol is mixed therewith. This adhesive, however, has disadvantages in that it is hard and brittle, and has poor flexibility, and that the volatile content at the time of heat-melting is high, resulting in poor heat stability.
Furthermore, Japanese Patent Application (OPI) No. 96831/1976 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a composition comprising polyvinyl alcohol and polyethylene glycol, and Japanese Patent Application (OPI) No. 96832/1976 describes that a composition comprising polyvinyl alcohol, an ethylene-vinyl acetate copolymer, and a plasticizer is useful as a water-soluble hot melt type of adhesive. In both compositions, however, polyvinyl alcohol having a low average degree of polymerization and a low degree of saponification, i.e., having many residual acetic acid groups is inevitably used in order to provide both water-solubility and hot melt properties thereto. For example, in Japanese Patent Application (OPI) Nos. 96831/1976 and 96832/1976, polyvinyl alcohol having an average degree of polymerization of 50 to 300 and containing 15 to 70 mol % of residual acetic acid group is used.
In general, the mechanical properties, such as tensile strength, of polyvinyl alcohol is reduced as the average degree of polymerization is lowered and as the amount of residual acetic acid groups is increases.
Adhesives, therefore, as disclosed in Japanese Patent Publication No. 9611/1974, and Japanese Patent Application (OPI) Nos. 96831/1976 and 96832/1976 wherein polyvinyl alcohol having a low average degree of polymerization and a large amount of residual acetic acid group are used have relatively poor adhesion strength. Thus, they are not suitable for use in applications wherein high adhesion strength is required.
In order to overcome the foregoing problems resulting from the use of water-soluble polyamides and vinyl alcohol-based polymers as a base polymer, the following methods may be used:
(1) The water-soluble polyamide or vinyl alcohol-based polymer per se is modified by techniques such as copolymerization and a chemical reaction. PA1 (2) The type and amount of reactive compounds, such as polymerizable vinyl monomers, added to the water-soluble polyamide or vinyl alcohol-based polymer are changed; and PA1 (3) The water-soluble polyamide or vinyl alcohol-based polymer is modified by adding thereto other polymers. PA1 (1) Some of the water-soluble polymers examined have poor compatibility with the water-soluble polyamide; that is, when they are mixed, aggregation occurs, yielding a precipitate and, therefore, difficulty is encountered even in uniformly mixing them; PA1 (2) Compounding of a mixture of a water-soluble polyamide and a vinyl alcohol-based polymer with, for example, polymerizable vinyl monomers and sensitizers provides a light-sensitive resin composition which is free from the defects of a light-sensitive resin composition prepared from the water-soluble polyamide or vinyl alcohol-based polymer alone; and PA1 (3) A mixture of a water-soluble polyamide and a vinyl alcohol-based polymer has both water-solubility and hot melt properties, and can provide, for example, a film having excellent adhesive properties, chemical stability under heating conditions, mechanical properties, and flexibility.
In accordance with Method (1), if it is desired to reduce the swelling properties of the water-soluble polyamide per se, for example, the object can be attained by increasing the crystallinity thereof. In order to increase the crystallinity of the water-soluble polyamide, it is necessary to change the structure of the water-soluble polyamide so that intermolecular hydrogen bonds are easily formed. For this purpose, it is required that the structures of the starting materials for the preparation of the water-soluble polyamide, e.g., dibasic acid, diamine and .omega.-aminocarboxylic acid, or the copolymerization composition is changed.
This method of increasing the crystallinity of the water-soluble polyamide by changing the structure thereof per se, however, not only reduces the water-solubility of the water-soluble polyamide, which is the greatest feature of the water-soluble polyamide, but also deteriorates the transparency of the water-soluble polyamide. This deterioration of the transparency leads to a reduction in the light-transmission of the water-soluble polyamide, and produces additional problems. For example, when a water-soluble polyamide having poor light-transmission is used as a substrate for a light-sensitive resin, the sensitivity is reduced.
In Method (1), the modification of the vinyl alcohol-based polymer (e.g., the provision of hot-melting properties) can be sometimes achieved by changing the degree of polymerization and degree of saponification of the polyvinyl alcohol, or by copolymerizing with an olefin, such as ethylene, or a vinyl monomer. In this case, however, it is very difficult to achieve the modification while maintaining the balance among the water-solubility, hot-melting properties, and mechanical properties. For example, the provision of hot-melting properties sometimes reduces the water-solubility and mechanical properties.
With regard to Method (2), wherein the type and amount of reactive compounds, such as polymerizable vinyl monomers, added to the water-soluble polyamide or vinyl alcohol-based polymer are changed, various investigations have heretofore been made. However, the type of reactive compounds, such as polymerizable vinyl monomers, which are compatible with water-soluble polyamides or vinyl alcohol-based polymers, is limited and, furthermore, the amount of reactive compounds which can be added is limited from a standpoint of solubility. Therefore, even though the type and amount of reactive compounds are changed within the possible ranges, improvements in the swelling characteristics of the water-soluble polyamide-based composition to water after the curing thereof and in the flexibility of the vinyl alcohol-based polymer composition are still insufficient.
Method (3), i.e., a method in which the water-soluble polyamide or vinyl alcohol-based polymer is modified by adding thereto polymers, has not heretofore received much attention. In performing the modification in accordance with Method (3), when a use for light-sensitive resin composition is considered, it is necessary for the mixture of the water-soluble polyamide or vinyl alcohol-based polymer and other polymers to have a transparency of the same level as that of the water-soluble polyamide or vinyl alcohol-based polymer in view of the light transmission.
Although it is generally believed that polymers having similar chemical structures have compatibility with each other, if a mixture or blend of different polymers is necessary to have transparency, it is required for the different polymers to have sufficient wetting in the interface therebetween to the extent that no scattered light is formed in the interface. It is, therefore, very difficult to presume only from the chemical structure of polymers as to if the blend of the polymers is transparent.
With regard to blends of polyvinyl alcohol and other water-soluble polymers, such as polyethylene glycol, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, soluble starch, polymethyl acrylate (20% saponified product), and polyethyl acrylate (20% saponified product), the stability in the state of an aqueous solution and the compatibility in the state of a film are described in C. A. Finch Ed., Polyvinyl alcohol, Properties and Applications, John Wiley & Sons (1973). However, there can be found no disclosure concerning a blend of polymers which has good compatibility over all mixing ratios and which provides a film having transparency.